The present invention relates to a radial tire for heavy duty vehicles which can restrain belt edge separation so as to improve high speed durability.
Heretofore, in a radial tire for such a heavy duty vehicle as a truck, bus and the like, a belt layer (b) composed of at least three belt piles (b1, b2, bn) of steel cords having the same modulus of elasticity to each other is arranged on a carcass (a), as shown in FIG. 5.
Further, in the belt layer (b), as shown in FIGS. 6 and 7, each cord of two belt piles (b1 and b2) laid radially outwardly is arranged in such a way that each cord angle against the equator is smaller than that of the innermost belt ply (bn) adjacent to the carcass (a), and the cords of belt piles (b1 and b2), in the case of n=3, cross with each other, and in the case of n=4, are arranged at the same direction. Thus the triangular structure is constituted as a whole so as to reinforce the tread part evenly with high rigidity.
However, it is well known that such a is subjected to the so-called ply separation in which the radially outermost belt ply (b1) separates from it's edges during high speed running so as to reduce high speed durability. This is caused because such a is often used under high load conditions so that tread the internal temperature becomes higher than a conventional tire. In addition, a belt layer (b1) is formed by cut end plies so that a restriction force on a tire case around the ply edge area becomes lower, as the result, due to an increase of the ground contact pressure accompanied with a growth in the outer diameter around the tread shoulder, whereby the tread internal temperature in such region further increases. In addition, because the outermost belt ply (b1), which directly receives the stress change from ground contact area is subjected to the most severe shearing strain, separation between belt plies b1 and b2 is accelerated.
Further, when the number of belt plies are three, each cord of the belt plies b1 and b2 are crossed with each other so that when a deforms, each cord extends to a different direction with respect to each other, and accordingly, a shearing stress acts between belt plies b1 and b2, whereby ply edge separation of belt ply (b1) is produced in cooperation with said increase of temperature at the shoulder region.
In order to prevent such ply separation, there are proposals that a rubber compound having low heat generation with a small loss fact is utilized as a tread rubber, or the tread rubber thickness gauge, in particular, the shoulder rubber gauge is reduced. However, such means cannot achieve an advantage effect but rather induces many problems on tire performance. For example, to adopt a rubber having low heat generation is to reduce road gripping performance, and to reduce tread rubber gauge which causes a deterioration in wear life and even wear.
Further, as the means to prevent such belt ply separation there is disclosed in Japanese patent application laid open No.59-67108 that, as shown in FIGS. 8 and 9, in the belt layer (b) composed of four steel belt plies b1, b2, b3 and b4, in order to prevent occurrence of shearing strain caused by difference of belt cord angle between second and third belt plies b2 and b3 counted from the tread side, the steel belt b3 is replaced by a belt ply composed of organic fiber cords to reduce ply separation between belt ply b2 and b3. In addition, both edges of the belt ply b2 are covered by organic fiber cords of the belt ply b3, thereby reducing the shearing strain at the edge of the belt ply b2 so as to prevent ply separation.
However, as regards an all steel tire comprising a carcass and a belt layer each composed of steel cords actually used for truck and bus tires, ply separation of the belt layer b is liable to occur between belt plies b1 and b2, because the belt ply b1 most adjacent to the ground contacting tread surface is most severely subjected to deformation from the ground. Accordingly, the prior art according to the patent publication can not prevent such ply separation.
On the other hand, Japanese patent application laid open No. 53-18105 teaches that ply edges of the outermost belt ply are covered by folding the edge parts of a belt ply laid adjacent and under the outermost belt ply. However, this publication is directed to a tire for a large vehicle used for off road operation. In addition, such a belt construction which is easy to deform in going with a rough road is adopted.
Accordingly, the prior art does not prevent ply separation of an outermost steel belt layer and does not improve high speed durability while maintaining tire performance of steel radial tires for truck and bus use.